1. Field of the Invention
The present invention relates to a method and a device for slitting a magnetic recording medium and, in particular, to such method and device which slide and rotate the respective cutting edges of upper and lower rotary blades to thereby slit a wide magnetic recording medium into a plurality of narrow magnetic recording mediums.
2. Description of the Related Art
Conventionally, in various fields, there are used a magnetic tape cassette such as an audio cassette, a video cassette and the like in which a long magnetic tape wound round a hub or the like is incorporated in a freely movable manner, and a magnetic disk cartridge in which a circular magnetic disk is movably stored. A magnetic recording medium including the magnetic tape, magnetic disk and the like is treated in the form of a long tape in its manufacturing process.
In manufacturing the magnetic recording medium, generally, a magnetic paint is coated on a plastic film and is then dried to form a magnetic layer. Alternatively, a ferromagnetic material is coated on a plastic film base by vacuum evaporation, sputtering or the like to form a magnetic layer, thereby producing a wide magnetic tape blank. After that, the wide magnetic tape blank is cut by a slitter into a magnetic tape having a given width.
A slitting device for slitting the magnetic tape blank is known in which, normally, a magnetic tape blank is moved between two or more pairs of an upwardly located disk-like upper rotary blade and a downwardly located disk-like lower rotary blade rubbing against each other. The magnetic tape blank is pressure slit by the upper and lower rotary blades to thereby manufacture a plurality of magnetic tapes each having a given width.
In a method using the above slitting device, the slitting speed is fast and the productivity is high. However, since the magnetic tape blank is slit from above by the upper rotary blade, there can be left, in the slit edge surface of the resultant magnetic tape having a given width, a slit trace extending in the longitudinal direction of the slit edge surface. Also, the magnetic layer may be removed from the slit edge surface. Additionally, when the magnetic tape is moved within a cassette or a recording and reproducing apparatus, the magnetic tape may contact the flange surface of a guide roller or the like to cause the removal of the magnetic layer and the shaving off of the plastic film base to thereby produce dropouts, so that the electromagnetic conversion characteristic of the magnetic tape may deteriorate.
Therefore, to solve the above-mentioned problems, several methods have been proposed. For example, in Japanese Patent Publication No. 62-202385 of Showa, there is disclosed a method in which the angle of a cutting edge of a cutting blade is set at a given angle with respect to a moving magnetic tape blank or the speed of rotation of a rotary blade is set in a given range.
However, in this method as well, when the cutting edge becomes dull, the surface quality of the cut portion deteriorates and, therefore, the cutting edge must be frequently replaced. The manufacturing line must be stopped for a relatively long time whenever the cutting edge is replaced.
Also, in Japanese Patent Publication No. 2-41892 of Heisei there is disclosed a structure of a slitting knife in which, in the finishing structure of a cutting edge of a disk-shaped upper rotary blade, the cutting edge of the upper rotary blade is formed in a recessed structure that allows the cutting edge of the upper rotary blade to contact a lower rotary blade at a given angle. That is, by working the cutting edge of the upper rotary blade to have a given angle in the recessing direction, even if the cutting edge is somewhat worn, the cutting quality of the cutting edge can be adequately maintained.
As described above, conventionally, there have been proposed several devices which aim at maintaining the cutting quality of the cutting edge even after it is used for a long period of time. In any of them, after the cutting edge is worn a given amount, the cutting edge must be whetted again to thereby keep the shape of the cutting edge at a given level at all times. If the cutting edge is worn further, then the worn cutting edge must be replaced with a new one. This inevitably results in operational losses and increases costs.